On the basis of dispersion relations for the magnetotelluric (MT) field,the physical meanings of 1-D joint inversion of MT data are expounded.The necessity and applied prospects of the joint inversion using a pair of MT response functions,especially the apparent resistivities of the impedance's real and imaginary parts,which are relative with the dispersion relations,are inferred.The effects of the single parameter's and joint inversions for MT impedance are tested.It is shown that in the aspects of increasing the velocity of the inverse iteration,limiting the non-uniqueness of the inverse solution and assuring the stable convergence of the inverse iteratinn,the joint inversions are more advahtageous than the single-parameter's inversions,in the most cases the joint inversion using the apparent resistivities of impedance's real and imaginary parts is more advantageous than the joint inversion using the normal apparent resistivity and impedance's phase,which is usually used now.The test for practical MT data shows that the use of the various average impedances and the synthetic interpretation of them will help to obtain more reliable results.